Wear resistant members have been applied to various fields, involving, i.e., bearing members, various roller members for rolling or the like, compressor vanes, gas-turbine blades, engine-parts such as cam rollers or the like, and so forth. For the above-described wear resistant members, ceramic materials have been used in addition to metallic materials. Especially, silicon nitride sintered bodies, which are excellent in mechanical strength and wear resistance, have been widely applied to various fields.
As conventional sintering compositions for silicon nitride sintered bodies, a silicon nitride/oxide of rare earth element (yttrium oxide or the like)/aluminum oxide system, a silicon nitride/oxide of rare earth element (yttrium oxide or the like)/aluminum oxide/titanium oxide system, a silicon nitride/yttrium oxide/aluminum oxide/aluminum nitride/oxides of titanium, magnesium, and zirconium system, and so forth are known. The addition of the sintering assistant agents of the oxide of rare earth elements to the above-described sintering compositions is carried so that grain boundary phases (liquid phase) composed of Si-rare earth element-Al—O—N and the like are formed during the sintering, and, thus, the densities and the strengths of the sintered bodies are enhanced.
For mass production of the known silicon nitride sintered bodies, the above-described sintering assistant agents as additives are added to powdery silicon nitride raw materials and molded, and the molded pieces are sintered in a firing furnace at a temperature of about 1,650 to 1,900° C. in a predetermined period of time.
The silicon nitride sintered bodies are ceramics having higher sliding characteristics compared to the other wear resistant members using the silicon nitride sintered bodies described above, and, thus, have been practically applied to bearing members, especially, to bearing balls. The bearings are used in various fields, and started to be reviewed to use as an important protection safety parts. For this reason, the reliabilities of the bearing members of silicon nitride sintered bodies, i.e., those of rolling members such as balls, rollers or the like are required to be further enhanced.
For example, defects such as flaws and cracks formed on a surface of the rolling member will result to a breakage of not only the bearing per se but also an entire system using the bearing member. Therefore, there is adopted a manufacturing process for excluding or eliminating such defects as completely as possible. In a similar way, pores or the like existing near the surface of the rolling body would be a cause of deteriorating the reliability of the bearing member. Thus, the pores are removed at a process when the bearing member is worked out into a product having a final shape.
The silicon nitride sintered body produced in the known method has a bending strength, a fracture toughness, and a wear resistance improved to some degree. However, the sintered body is an electrical insulator. Therefore, example, in the case in which the sintered body is used as a bearing ball in the rotary portion of a hard disc drive device (HDD), the static electricity generated when the bearing ball is rotated at a high speed could not be effectively dissipated into the ball bearing portion. Thus, undesirably a large quantity of static electricity may be time-dependently accumulated. Thus, it has been observed that problems occur in that the hard disc drive device (HDD) can not normally be operated.
On the other hand, silicon nitride sintered bodies having a low electric resistance, i.e., a electric resistance of about 10−2 Ω·cm are known, and are mainly used as a cutting tool or the like. However, to render a low electric resistance, a large amount of conductivity-rendering particles of carbides or the like are added. Thus, the particles tend to aggregate with each other. Thus, problems occur in that the bending strength and the fracture toughness are easily deteriorated. Moreover, in the case where the silicon nitride sintered bodies are applied to bearing balls or the like which suffer compression loads from all of the directions thereof, problems occur in that cracks tend to be formed in the sites where large number of aggregate particles described above exist, so that the sliding characteristic decreases is a short time. Accordingly, for the sintered bodies which are used under the condition that they suffer a compression load from all of the directions thereof, it is desired that the number of the aggregate particles is as small as possible.
To solve the above-described problems, the present invention has been devised. It is an object of the present invention to provide a silicon nitride type wear resistant member which has a predetermined electric resistance (electroconductivity) in addition to a high strength and fracture toughness inherent in the silicon nitride per se, and especially has a superior sliding characteristic and to provide a method of producing the member.